An active matrix liquid crystal display which is driven by a thin film transistor (TFT) is used for a notebook-type personal computer in which the thinness of the display and low power consumption are critical. At present, the TN-type liquid crystal is used for the active matrix liquid crystal display. However, there are some problems. These include its field angle dependence and delayed response to motions. To resolve these problems, various liquid crystal materials and display modes have been studied.
An anti-ferroelectric liquid crystal which does not have a threshold value as a liquid crystal provides a wide field angle because the liquid crystals move within a cell plane. Its spontaneous polarization is able to increase the speed of liquid crystal motion by means of an electromagnetic field control. This is effective in increasing optical response speed and the resulting display performance.
Assume that an anti-ferroelectric liquid crystal is driven by the active matrix technique. When there are many pixels, that is, many scanning lines, the response speed (several tens of microseconds) of the liquid crystal is faster than the time frame cycle. However, compared to the time required for selecting one line, it is slower. As a result, when the TFT is turned on, the liquid crystal is not able to move freely but instead, the liquid crystal molecules try to move by the voltage charged when the TFT is turned off. While this is not a problem for a normal TN-type liquid crystal, a problem was observed for an anti-ferroelectric liquid crystal. That is, anti-ferroelectric liquid crystal molecules move by spontaneous polarization. Electrons, charged to the storage capacitance of the liquid crystal when the TFT is turned on, flow into the liquid crystal side when the TFT is turned off. As a result of this phenomenon the pixel voltage decreases.
Because of this phenomenon, the voltage applied to the liquid crystal decreases, decreasing the contrast and response speed, causing poor display properties. Also, the amount of voltage decrease from the original signal voltage of the pixel is dependent on the response speed of the liquid crystal, the resistivity of a conducting transistor, and writing time. A non-uniform display was observed when temperature distribution from back lighting exists within the screen or when the gate has a large resistivity and the gate pulse width changes due to the distance between the gate and the power supply end. It is understood this is due to the phenomena mentioned above.
This phenomenon was observed for different liquid crystals, besides anti-ferroelectric liquid crystals. Such crystals have a large induction and change the induction levels due to the liquid crystal molecule movement, such as cholesteric liquid crystals and the like.
As described, a problem with the prior art is that the liquid crystal cannot respond completely within the selected time when the matrix is driven. Therefore the display is degraded.
Another problem was observed in the liquid crystal display using, for example, the TN-type liquid crystal and the like, which requires that the drive force be derived from an alternating current frequency source to eliminate flicker (e.g., 60 Hz). Therefore, even when the same display image is kept, the display is driven by alternating current, consuming unnecessary power.